Donald Trump says firms doing business in Iran to be barred from US

'These are the most biting sanctions ever imposed, and in November they ratchet up to yet another level'

Firms doing business with Iran will be barred from the United States, President Donald Trump said on Tuesday, as new US sanctions took effect in spite of pleas from Washington's allies.

Iran dismissed a last-minute offer from the Trump administration for talks, saying it could not negotiate while Washington had reneged on a 2015 deal to lift sanctions in return for curbs on Iran's nuclear programme.

Trump decided this year to pull out of the agreement, ignoring pleas from the other world powers that had co-sponsored the deal, including Washington’s main European allies Britain, France and Germany, as well as